In a revolving-field type synchronous generator, a direct-current is provided to a generator field circuit, which causes field coils in the generator to become an electromagnet.
The type of the method of exciting the field coils is roughly divided into a thyristor excitation system and a brushless excitation system. In the brushless excitation system, a sliding contactor such as a brush or a slip ring which electrically connects the brush is not necessary.
As the synchronous generator of the brushless excitation system doesn't require a sliding contactor, and thus the replacement of a brush and frequent maintenance are unnecessary. Hence, a synchronous generator of the brushless excitation system is actively adopted in, e.g., Europe.
The brushless excitation system includes an alternating-current. (AC) exciter configured to mainly excite a field circuit of a field coils and a permanent magnet generator (PMG) configured to excite this AC exciter.
An AC exciter armature rotates in a static magnetic field which is generated in an enclosed space of an AC exciter field coil by the PMG, and thereby an AC current is generated in the AC exciter armature.
This AC current is rectified into a direct current by a rectifier circuit provided on a rotational shaft, then the direct current flows into the generator field circuit, and thereby the generator field coil is excited.
In the brushless excitation system, it is possible to control the mount of the current in the generator field circuit and to control the excitation of the generator field coil by increasing or decreasing the current flowing from a PMG armature into the AC exciter field circuit.
In general, the amount of the power generated is kept constant by an automatic voltage regulator (AVR) which automatically regulates the output of the PMG by monitoring a generator output voltage.
When an accident occurs in an electric power system, e.g., when a generator output voltage rapidly drops due to the cloud-to-ground discharge, the AVR controls the output of the PMG and raises the output voltage of the generator in a short time by applying a field voltage which is higher than a rated field voltage of the generator (hereinafter, this operation is referred to as a positive forcing operation).
Although an output voltage of the generator is stably controlled by a conventional AVR even in the case of irregularity as described above, there has been a demand for novel technology to achieve to control the output voltage of the generator more reliably than conventional technology.